First you need to find the following line and remove the comment from the file "/etc/ssh/sshd_config".
ie. Just remove the preceding "#"
You can edit it in your favorite editor. But remember you need to be have root access to edit it.
Or simply you can run the following command from terminal
sudo sed -i "s/#PermitRootLogin/PermitRootLogin/g" /etc/ssh/sshd_config
Now, For this to take effect, you need to restart the service:
sudo /etc/init.d/sshd restart